One of the fundamental decisions an animal must make is whether to ingest or reject a chemical. While in some species such discriminations are made visually, it is through the chemical senses that most animals choose their foods. The choice must involve two aspects: (1) is this chemical nutritious? Will it fulfill some bilogical need such as energy or protein production or the maintenance of electrolyte balance? (2) is it harmful? Will it interfere with normal biochemical processes because of its pH or molecular structure? Since it is advantageous to consume nutritious chemicals and to avoid those that are harmful, the evolutionary process should have shaped gustation for this purpose. Nutrients ought to taste good; toxins, regardless of molecular structure, should taste bad. Moreover, taste, through its ramifications in the limbic sytem, is imbued with powerful hedonic attributes such that rejection is accompanied by revulsion and acceptance by pleasure. The aims of the proposed research are to establish the role of toxicity in the neural code for taste and to begin the analysis of higher-order gustatory responses in the brainstem. Specifically, we will (1) discover whether stimulus toxicity, judgments of which are competently managed in the brainstem, is a major determinant of the gustatory neural code. This we will do through single neuron recordings in the medulla and pons in response to a series of chemicals varying in toxicity and in physicochemical attributes (pH, molecular weight, hy drophobicity), (2) establish the sensitivities and response characteristics of brainstem taste neurons. These not only signal stimulus quality and intensity, they are also thought to coordinate taste with body needs and with related sensory input (olfaction, soimesthesis). In addition, they may manage gustatory learning processes (conditioned taste aversions and preerences) and the hedonic concomitants of taste. For this we will record from awake unrestrained rats fitted with moveable microelectrodes. After determining the basic characteristics of these rats' taste cells, we will manupulate the chemical environment of the body through intravenous injections of nutrients, hormones, electrolytes and water to study the effect of body needs on diencephalic taste cell activity.